Not applicable.
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1. Field of the Invention
The present invention relates to a Process for Making Rare Earth Doped Optical Fiber.
2. Description of the Related Art
Rare-earth (RE) doped optical fibers have shown great potential for a number of applications including amplifiers, fiber lasers and sensors. Oxides of rare earths are doped into the core of such fibers as the active substance. Lasing and amplification have been demonstrated at several wavelengths with the incorporation of various rare-earths but for telecommunication applications erbium doped fiber (EDF) remains the most important since the operating wavelength matches with the third low loss optical window.
Erbium doped fiber amplifier (EDFA) operating around 1.53 xcexcm low loss window is playing the key role in the present day high capacity communication systems. It is able to amplify the optical signal directly independent of modulation format. Optoelectronic repeaters so long used in these systems were 3R devices with the limitations of amplifying the signal in discrete wavelengths. EDFA has the capability to amplify simultaneous optical channels in a single fiber, which has enabled the implementation of WDM (wavelength division multiplexing) technology with the potential of increasing the bandwidth of long distance transmission systems from Gb/s to Tb/s ranges. It thus exhibits high gain, large bandwidth, low noise, polarization insensitive gain, substantially reduced cross talk problems and low insertion losses at the operating wavelengths. The success of future high capacity optical networking and transmission systems will depend significantly on the development of efficient EDFA.
Reference may be made to Townsend J. E., Poole S. B., and Payne D. N., Electronics Letters, Vol. 23 (1987) p-329, xe2x80x98Solution-doping technique for fabrication of rare-earth-doped optical fiberxe2x80x99 wherein, the MCVD process is used to fabricate the preform with a step index profile and desired core-clad structure while solution doping is adopted for incorporation of the active ion. The steps involved in the process are as follows:
i. A conventional cladding doped with P2O5 and F is deposited within a high silica glass substrate tube to develop matched clad or depressed clad type structure.
ii. The core layers of predetermined composition containing index-raising dopant like GeO2 are deposited at a lower temperature to form unsintered porous soot.
iii. The tube with the deposit is immersed into an aqueous solution of the dopant precursor (typical concentration 0.1 M) up to 1 hour. Any soluble form of the dopant ion is suitable for preparation of the solution although rare earth halides have been mostly used.
iv. Following immersion, the tube is rinsed with acetone and remounted on lathe.
v. The core layer containing the RE is dehydrated and sintered to produce a clear glassy layer. Dehydration is carried out a temperature of 600xc2x0 C. by using chlorine. The level of OHxe2x88x92 is reduced below 1 ppm using Cl2/O2 ratio of 5:2 provided the drying time exceeds 30 min.
vi. Collapsing in the usual manner to produce a solid glass rod called preform.
vii. Fiber drawing is conventional.
Reference may also be made to DiGiovanni D. J., SPIE Vol. 1373 (1990) p-2 xe2x80x9cFabrication of rare-earth-doped optical fiberxe2x80x99 wherein the substrate tube with the porous core layer is soaked in an aqueous or alcoholic solution containing a nitrate or chloride of the desired RE ion. The tube is drained, dried and remounted on lathe. The dehydration is carried out by flowing dry chlorine through the tube at about 900xc2x0 C. for an hour. After dehydration, the layer is sintered and the tube is collapsed to be drawn to fiber.
Another reference may be made to Ainslie B. J., Craig S. P., Davey S. T., and Wakefield B., Material Letters, Vol. 6, (1988) p-139, xe2x80x9cThe fabrication, assessment and optical properties of high-concentration Nd3+ and Er3+ doped silica based fibersxe2x80x9d wherein optical fibers based on Al2O3xe2x80x94P2O5. xe2x80x94SiO2 host glass doped with high concentrations of Nd3+ and Er3+ have been fabricated by solution method and quantified. Following the deposition of cladding layers P2O5 doped silica soot is deposited at lower temperature. The prepared tubes are soaked in an alcoholic solution of 1M Al(NO3)3+various concentrations of ErCl3 and NdCl3 for 1 hour. The tubes are subsequently blown dry and collapsed to make preforms in the usual way. Al is said to be a key component in producing high RE concentrations in the core centre without clustering effect. It is further disclosed that Al and RE profile lock together in some way, which retards the volatility of RE ion. The dip at the core centre is observed both for P and GeO2.
Reference may also be made to U.S. Pat. No. 5,005,175 (1991) by Desuvire et al., xe2x80x98Erbium doped fiber amplifierxe2x80x9d wherein the fiber for the optical amplifier comprises a single mode fiber doped with erbium in the core having a distribution profile of the RE ion whose radius is less than 1.9 xcexcm while the radius of the mode of the pump signal exceeds 3 xcexcm. The numerical aperture (NA) of the fibers varies from 0.2 to 0.35 and the core is doped with both Al and Ge oxides to increase the efficiency. As the radius of the Er doped core region is equal to or less than the radius of the pump mode of the fiber it is claimed that each atom of erbium in the core cross section is exposed to substantially equal levels of the high intensity portion of the pump mode. The fiber with such design is reported to have increased gain and lower threshold compared to the conventional Er doped fiber amplifiers where the radius of the Er doped core is large compared to the radius of the pump mode so that the erbium atoms at the edge of the core do not see a sufficient flux of the pump photons to yield a net gain.
According to U.S. Pat. No. 5,491,581 (1996) by G. S. Roba, xe2x80x98Rare earth doped optical fiber amplifiersxe2x80x99 wherein high germania concentration in the core used to enhance the NA of the fiber is reported to result in generation of residual stress at the core-clad interface due to difference in viscosity and thermal expansion coefficient. Residual stress in turn is believed to produce undesirable increase in background loss of the fiber.
According to U.S. Pat. No. 5,778,129 (1998) by Shukunami et. al., xe2x80x98Doped optical fiber having core and clad structure are used for increasing the amplification band of an optical amplifier using the optical fiberxe2x80x99 wherein the porous core layer is deposited after developing the cladding inside a quartz tube by MCVD process and solution doping method is employed to impregnate Er as the active ion into the porous core to be followed by vitrification and collapsing for making the preform. The solution also contains compound of Al, say chlorides, for codoping of the core with Al in order to expand the amplification band. The Er and Al doped glass constitutes first region of the core. Surrounding this are the second and third regions of the core. The third region contains Ge to increase the refractive index. The second region has an impurity concentration lower than both those of first and third regions and consequently low RI also. The second region acts as a barrier to prevent diffusion of the active dopant.
Reference may also be made to U.S. Pat. No. 5,474,588 (1995) by Tanaka, D. et. al., xe2x80x98Solution doping of a silica with erbium, aluminum and phosphorus to form an optical fiberxe2x80x99 wherein a manufacturing method for Er doped silica is described in which silica glass soot is deposited on a seed rod (VAD apparatus) to form a porous soot preform, dipping the said preform into an ethanol solution containing an erbium compound, an aluminum compound and a phosphoric ester, and desiccating said preform to form Er, Al and P containing soot preform. The desiccation is carried out for a period of 24-240 hours at a temperature of 60xc2x0-70xc2x0 C. in an atmosphere of nitrogen gas or inert gas. This desiccated soot preform is heated and dehydrated for a period of 2.5-3.5 hours at a temperature of 950xc2x0-1050xc2x0 C. in an atmosphere of helium gas containing 0.25 to 0.35% chlorine gas and further heated for a period of 3-5 hours at a temperature of 1400xc2x0-1600xc2x0 C. to render it transparent, thereby forming an erbium doped glass preform. The segregation of AlCl3 in the preform formation process is suppressed due to the presence of phosphorus and as a result the doping concentration of Al ions can be set to a high level ( greater than 3 wt %). The dopant concentration and component ratio of Er, Al and P ions are claimed to be extremely accurate and homogeneous in the radial as well as in longitudinal directions.
A few of the drawbacks of the above mentioned processes are as follows:
1. Step like RE distribution profile is obtained in the core resulting to poor overlap between the pump signal and the RE ions which lowers the pump efficiency.
2. Step like RE distribution requires high numerical aperture (NA) of the core or confinement of the RE in the central region (say 50% of the total core area) for increase in pump efficiency which in turn leads to the following disadvantages:
i) Doping of RE only in selected portion of the core is extremely difficult and affects the repeatability of the process due to the sensitivity of the method to process parameters during various stages of processing such as deposition, solution doping, drying and sintering.
ii) Increasing the NA of the fiber with simultaneously reducing the core area requires high germania concentration in a small core which enhances the possibility of formation of the dip at the centre due to evaporation during sintering and collapsing.
iii) For preforms with high NA ( greater than 0.20) high germania concentration in the core lowers the viscosity of the glass and makes the process very sensitive to temperature especially during the stages of porous soot layer deposition and sintering.
iv) Increase in temperature sensitivity during porous soot deposition leads to variation in composition and soot density along the length of the tube.
v) High germania concentration in the core results to generation of residual stress at the core-clad interface due to difference in viscosity and thermal expansion coefficient. Residual stress produces undesirable increase in background loss of the fiber.
vi) Residual stress is believed to introduce polarization mode dispersion (PMD) which results in serious capacity impairments including pulse broadening. Since the magnitude of PMD at a given wavelength is not stable passive compensation becomes impossible.
3. Dehydration and sintering of the RE chloride containing soot layer is critical because it alters the composition by vaporization and also diffusion of the dopant salt as well as GeO2 present in the core.
The main object of the present invention is to provide a process for making Rare Earth doped optical fiber, which obviates the drawbacks as detailed above. Another object of the present invention is to provide fibers possessing controlled distribution of RE, more particularly Erbium in the doped region similar to the pump beam intensity distribution in the fiber with maximum concentration at the centre so that the overlapping between the two is considerably improved.
Still another object of the present invention is to provide fibers in which the pump beam has a radius of distribution equal to or greater than the radius of distribution of RE ions in the core to increase the chances of all the active ions getting exposed to the pump light, consequently increasing the pump conversion efficiency in the fiber.
Yet another object of the present invention is to provide a method of controlling the Gaussian RE distribution profile along the radial direction in the core.
Still another object of the present invention is to achieve high optical gain in the fibers for NA value close to 0.20 only thus avoiding wide variation in composition between the core and cladding glass to eliminate problems like residual stress and PMD.
Yet another object of the present invention is to develop erbium doped fibers suitable for amplification of the input signal with NA and mode field diameter not widely different from signal transmitting fiber for ease of splice.
Still another object of the present invention is to reduce the possibility of change in composition of the particulate core layer due to evaporation of the RE salt during drying and sintering.
Yet another object of the present invention is to reduce the quantity of germanium halide required to achieve the desired NA in the fiber.
One more object of the present invention is to provide a process where the numerical aperture of the fiber is varied from 0.10 to 0.30 maintaining RE concentration in the core between 50 to 6000 ppm along with variation in RE distribution profile in the doped region to produce fibers suitable for application as amplifiers, fiber lasers and sensors for different purposes.
The novelty of the present invention lies in controlling the concentration profile of RE ion in the collapsed preform by minimizing evaporation of the RE salt and also preventing diffusion of the rare earth ion due to subsequent heat treatment. The optimum soot density to achieve this objective is estimated to lie between 0.3 to 0.5 after deposition. The inventive step lies in transformation of the RE salts to oxides by gradually heating the tube to a higher temperature maintaining an oxidizing atmosphere inside, thereby minimizing the possibility of evaporation of RE during subsequent processing as the oxide has a very high melting temperature compared to halide/nitrate salts. This step also helps to remove the solvent trapped within the porous layer. The inventive step also includes increasing the temperature of the RE containing porous layer gradually in steps of 50 to 200xc2x0 C. up to the sintering temperature and above for sintering and further fixing of the RE ions in their desired sites. The steps will depend on the host glass composition and Er/Al concentration of the core layer. The incorporation efficiency of the RE from the solution to the core layer thus increases appreciably making the process more efficient and economic. The RE distribution along the transverse direction in the core will depend on the density of the porous soot layer, dipping period and the processing conditions during oxidation, sintering and collapsing.
The sintering of the porous core layer in GeO2 rich atmosphere along with the addition of oxygen and helium is another inventive step of the process which reduces the quantity of GeCl4 required to achieve the desired NA and adds to the economy of the process. At temperatures between 200xc2x0 to 1400xc2x0 C. during the sintering step pure GeCl4 is supplied with the input oxygen, the quantity of which depends on the NA desired in the fiber. The sintering is continued by gradually raising the temperature till a clear glassy layer is formed.